Solid golf ball

ABSTRACT

In a solid golf ball comprising a solid core and a cover, provided that a coefficient of restitution at a firing velocity of v m/s is designated by CORv, a ball structure prior to formation of the cover satisfies COR25-COR50≧0.100, and the ball satisfies COR50≧0.740 and COR25-COR50≧0.09. The golf ball&#39;s performance depends little on the head speed so that the flight performance of the ball when hit at a low head speed is improved while the flight performance when hit at a high head speed is little deteriorated.

BACKGROUND OF THE INVENTION

[0001] On solid golf balls having solid cores, a number of attempts havebeen made to soften the feel of the ball when hit. One common approachis to soften the core. At present, this approach requires to use arelatively hard cover in order to compensate for a loss of resilience.Undesirably, the hard cover tends to reduce the substantial dependencyof resilience on speed that the soft core possesses.

[0002] In general, the substantial dependency of resilience on speed ofgolf balls means that the resilience at high head speeds is low, but avery good resilience is exerted in a low head speed region. Such ballsare suited for low head speed players.

[0003] The soft core technology known thus far, however, failed toimpart to the ball a fully high resilience at low head speeds because ofthe influence of the hard cover.

SUMMARY OF THE INVENTION

[0004] An object of the invention is to provide a solid golf ball havingimproved flight performance in a low head speed region and a soft feelwhen hit.

[0005] The invention is directed to a solid golf ball comprising a solidcore and a cover enclosing the core. It was empirically found that ifthe firing velocity used in the measurement of a coefficient ofrestitution (COR) and the head speed (HS) upon hitting with a given clubare of the same value, the deformation behaviors of the ball upon impactare substantially identical between the measurement of COR and the clubhitting. It has been found that by specifying the COR at a firingvelocity of the golf ball and the difference of COR in a low HS region(typically 25 m/s) and a high HS region (typically 50 m/s) of both theball and a ball structure prior to formation of the cover, quiteunexpectedly there is obtained a golf ball which is improved in flightperformance in the low head speed region without sacrificing the flightperformance in the high head speed region and at the same time, has asoft feel when hit.

[0006] It has also been found that by specifying the Shore D hardness ofthe cover material relative to the difference in COR of the ballstructure prior to formation of the cover between the low and high HSregions, the gage of the cover, and the contact area of the ball whenhit at a velocity of 50 m/s, the speed dependency of resilience of theball when hit at a relatively low head speed can be enhanced and theabove-mentioned advantages are more effectively achievable withoutsacrificing the resilience or rebound in the high head speed region.

[0007] Accordingly, the invention provides a solid golf ball comprisinga solid core and a cover. Provided that a coefficient of restitution ata firing velocity of v m/s is designated by CORv, a ball structure priorto formation of the cover satisfies COR25- COR50 ≧0.100, and the ballsatisfies COR50≧0.740 and COR25 -COR50 ≧0.09.

[0008] In one preferred embodiment, the cover is formed to a gage of 1.2to 3.0 mm and of such a material that the Shore D hardness of the coveris not greater than 327.61X +27.239 wherein X represents (COR25- COR50)of the ball structure prior to formation of the cover. Preferably, theball structure prior to formation of the cover undergoes a deflection of2.8 to 5.0 mm under an applied load of 100 kg. Also preferably, thesolid golf ball has a contact area of 5.0 to 6.5 cm² when fired at afiring velocity of 50 m/s.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The solid golf ball of the invention has at least a sphericalsolid core and a cover enclosing the core in a concentric fashion. Therespective components may be formed of well-known materials byconventional methods.

[0010] The core may be formed of well-known rubber compositions. Forexample, a rubber composition comprising polybutadiene as the base ispreferred. The polybutadiene used herein is preferablycis-1,4-polybutadiene having a cis structure of at least 40%. Wheredesired, another suitable rubber component such as natural rubber,polyisoprene rubber or styrene-butadiene rubber may be compounded withthe polybutadiene to give the base rubber. Increasing the polybutadienecomponent leads to an improvement in resilience. Preferably less thanabout 10 parts by weight of the other rubber component is blended per100 parts by weight of polybutadiene.

[0011] A crosslinking agent may be included in the rubber composition.Exemplary crosslinking agents are the zinc and magnesium salts ofunsaturated fatty acids, such as zinc dimethacrylate and zincdiacrylate, and ester compounds such as trimethylpropane methacrylate.Zinc diacrylate is especially preferred for achieving a high resilience.The crosslinking agent is preferably included in an amount of about 10to about 40 parts by weight per 100 parts by weight of the base rubber.

[0012] A vulcanizing agent is generally compounded in the rubbercomposition. Peroxides are preferred vulcanizing agents. It isrecommended that the vulcanizing agent include a peroxide having a oneminute half-life temperature of not higher than 155° C., and suchperoxide account for at least about 30%, more preferably about 40 to 70%by weight of the entire vulcanizing agent. Examples of suitableperoxides include commercially available products such as Perhexa 3M(dicumyl peroxide, manufactured by Nippon Oils and Fats Co., Ltd.). Theamount of vulcanizing agent included in the rubber composition ispreferably from about 0.6 to about 2 parts by weight per 100 parts byweight of the base rubber.

[0013] If necessary, other suitable ingredients may also be incorporatedin the rubber composition, such as antioxidants and fillers (e.g., zincoxide, barium sulfate) for modifying the specific gravity. The amount ofthe gravity adjuster blended is typically about 1 to 30 parts by weightper 100 parts by weight of the base rubber.

[0014] Production of the core from the rubber composition may be carriedout by a known method involving molding and vulcanizing or curing steps.

[0015] Around the core, an intermediate layer can be formed from asimilar rubber composition or a thermoplastic resin base composition.Specifically, the intermediate layer may be formed by using a suitablematerial selected from among ionomer resins and thermoplastic elastomerssuch as polyurethane elastomers, polyamide elastomers and polyesterelastomers, and molding the material around the core by a conventionalinjection molding process. The intermediate layer may be formed to asingle layer or a multilayer structure of two or more layers.

[0016] As used herein, the term “ball structure prior to formation ofthe cover” means the core itself when the golf ball has a two-layerstructure consisting of a core and a cover. When the golf ball furtherhas an intermediate layer between the core and the cover as mentionedjust above, a sphere having the core enclosed with the intermediatelayer is meant by the ball structure.

[0017] The ball structure prior to formation of the cover preferably hasa deflection of 2.8 to 5.0 mm, more preferably 3.0 to 4.8 mm, and mostpreferably 3.0 to 4.6 mm, under an applied load of 100 kg. A deflectionof less than 2.8 mm may lead to a too high hardness and make itdifficult to increase the speed dependency of resilience. A deflectionin excess of 5.0 mm may lead to poor resilience.

[0018] Though not critical, the ball structure prior to formation of thecover generally has an outer diameter of 36.7 to 40.3 mm, and especially37.5 to 39.8 mm.

[0019] The ball structure prior to formation of the cover may be asingle layer (that is, the core alone) or have a multilayer structure oftwo or more layers (that is, the core enclosed with the intermediatelayer or layers), with the multilayer structure being preferred. In themultilayer embodiment, the ball structure as a whole may fall within theabove-indicated ranges of the deflection under 100 kg load and the outerdiameter. Where the intermediate layer is formed, it preferably has athickness or gage of 0.5 to 5.0 mm, and especially 1.0 to 4.5 mm.

[0020] According to the invention, the ball structure prior to formationof the cover and the ball each should have a specific difference betweenthe coefficients of restitution (COR) at predetermined firingvelocities.

[0021] Coefficient of restitution (COR) is measured by firing a golfball or a ball structure prior to formation of the cover in a pneumaticcannon at a velocity (referred to as firing velocity) against a steelplate which is positioned apart from the muzzle of the cannon. Therebound velocity is then measured. The rebound velocity is divided bythe forward velocity to give the coefficient of restitution. A COR valuewhich is more approximate to unity (1) indicates higher resilience. Theinvention uses the nomenclature that a coefficient of restitution at afiring velocity of v m/s is designated by CORv. That is, COR's at firingvelocities of 25 m/s and 50 m/s are designated COR25 and COR50,respectively.

[0022] According to the invention, the ball structure prior to formationof the cover should satisfy COR25-COR50 ≧0.100, preferably COR25-COR50≧0.105, and more preferably COR25-COR50 ≧0.110. If this difference isless than 0.100, the speed dependency of resilience of the ball cannotbe enhanced.

[0023] Specifically stated, it is recommended that the ball structureprior to formation of the cover have a COR value of 0.80 to 0.90,especially 0.82 to 0.88 at a firing velocity of 25 m/s, and 0.70 to0.80, especially 0.72 to 0.78 at a firing velocity of 50 m/s. Outsidethe ranges, it may be difficult to provide the desired differencebetween COR25 and COR50.

[0024] The golf ball of the invention is obtained by forming the coveraround the ball structure prior to formation of the cover (i.e., thecore alone or the core enclosed with the intermediate layer). Accordingto the invention, the ball should satisfy COR25-COR50 ≧0.090, preferablyCOR25-COR50≧0.095, and more preferably COR25-COR50≧0.100. If thisdifference is less than 0.090, the speed dependency of resilience of theball cannot be enhanced.

[0025] Additionally, the invention requires that the ball have a COR ofat least 0.740 at a firing velocity of 50 m/s (that is, COR50 ≧0.740),preferably at least 0.745 and more preferably at least 0.750 at a firingvelocity of 50 m/s. The COR of the ball at a firing velocity of 25 m/sneed not be specifically limited although the ball preferably have aCOR25 of at least 0.830, more preferably at least 0.835, and mostpreferably at least 0.840. Outside the ranges, it may be difficult toprovide the optimum difference between COR25 and COR50 and the desiredspeed dependency of resilience of the ball.

[0026] The golf ball of the invention is prepared by enclosing the coreor the core plus the intermediate layer with the cover. The cover may bemade of well-known cover stocks such as thermoplastic resins. Suitablematerials include ionomer resins and thermoplastic elastomers such aspolyurethane elastomers, polyamide elastomers and polyester elastomers.

[0027] In forming the cover, a choice is preferably made of a materialhaving an appropriate hardness for the predetermined difference ofcoefficient of restitution of the ball structure prior to formation ofthe cover. Provided that X represents (COR25-COR50) of the ballstructure prior to formation of the cover and D represents the Shore Dhardness of the cover material, it is recommended that the covermaterial satisfy

[0028] D≦327.61X+27.239. Specifically, the cover material shouldpreferably have a Shore D hardness of 50 to 65, and especially 53 to 62.A hardness outside the range may detract from the feel and fail toprovide good spin performance enough for skilled golfers to accept.

[0029] The thickness or gage of the cover is not critical although thecover gage is generally 1.2 to 3.0 mm, preferably 1.5 to 2.5 mm, andespecially 1.6 to 2.1 mm. A gage of less than 1.2 mm may detract fromball durability. A cover gage of more than 3.0 mm may offset the speeddependency of resilience of the core, failing to attain the objects ofthe invention.

[0030] Most often, the cover is formed by injection molding of theabove-described materials. Of course, other well-known moldingtechniques such as compression molding are employable.

[0031] In the practice of the invention, the golf ball satisfying theabove-mentioned requirements of COR and COR difference can be obtainedby properly selecting the material type, vulcanizing conditions anddeflection of the core, the material type, hardness and gage of theoptional intermediate layer, and the material type, hardness and gage ofthe cover.

[0032] The golf ball of the invention can be prepared by injectionmolding the above-described cover material around the ball structureprior to formation of the cover in a conventional manner. Likeconventional golf balls, the golf ball of the invention has numerousdimples formed in the surface of the cover. The shape and arrangement ofdimples are selected as appropriate from well-known ones.

[0033] The golf ball of the above construction preferably has adeflection of 2.4 to 3.8 mm and especially 2.6 to 3.5 mm under anapplied load of 100 kg.

[0034] Further preferably, the golf ball has a contact area of 5.0 to6.5 cm², more preferably 5.2 to 6.3 cm², and most preferably 5.4 to 6.0cm², when fired at a firing velocity of 50 m/s. A contact area of lessthan 5.0 cm² may lead to a hard feel whereas a contact area of more than6.5 cm² may detract from resilience.

[0035] The inventive golf ball may be formed so as to have a diameterand weight which conform with the Rules of Golf, that is, a diameter ofnot less than 42.67 mm and a weight of not greater than 45.93 g.

EXAMPLE

[0036] Examples of the invention are given below by way of illustrationand not by way of limitation.

Examples 1-4 and Comparative Examples 1-4

[0037] Using the materials of the formulation shown in Table 1, coreswere formed, and optional intermediate layers and covers were formedaround the cores by injection molding. In this way, there were obtainedgolf balls bearing identical dimples.

[0038] The golf balls were examined as follows, with the results shownin Table 1.

Flight distance

[0039] Using a swing robot, the ball was hit with a driver (W#1, tradename “230Ti” with a loft 9.5°, by Bridgestone Sports Co., Ltd.) at ahead speed of 35 m/s (HS35) and 50 m/s (HS50). A carry and a totaldistance (carry plus run) were measured.

COR

[0040] Coefficient of restitution (COR) was measured by firing the ballin a pneumatic cannon at a velocity of 25 m/s or 50 m/s against a steelplate which is positioned apart from the muzzle of the cannon. Therebound velocity was then measured. The rebound velocity is divided bythe forward velocity to give the COR.

Contact area

[0041] As in the measurement of COR, the ball was fired at a velocity of50 m/s against a pressure-sensitive paper sheet on the steel plate. Acontact area was determined from the imprint on the paper by the ball.

Feel

[0042] The feel of the golf ball when hit with a driver (W#1) was ratedas follows by three golfers.

[0043] Good: Three golfers got an appropriate soft and solid feel.

[0044] Fair: Two golfers got an appropriate soft and solid feel.

[0045] Poor: Three golfers felt too soft. TABLE 1 EX 1 EX 2 EX 3 EX 4 CE1 CE 2 CE 3 CE 4 Core Cis-1,4- 100 100 100 100 100 100 100 100formulation polybutadiene (pbw) Zinc diacrylate 25.3 18.4 22.2 13.0 23.820.7 11.5 34.5 Barium sulfate 17.3 20.4 18.7 29.6 18.0 22.8 23.5 13.2Zinc oxide 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 Antioxidant 0.2 0.2 0.2 0.20.2 0.2 0.2 0.2 Dicumyl peroxide 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2Intermediate Himilan 1557 — — — 50 — — — — layer Himilan 1601 — — — 50 —— — — formulation Hytrel 4767 — — 100 — — — 100 — (pbw) Cover Himilan1601 — — — 50 — — — 50 formulation Himilan 1557 — 50 — 50 — 50 50 50(pbw) Himilan 1605 50 50 50 — — 50 50 — Himilan 1706 50 — 50 — 50 — — —Surlyn 8150 — — — — 50 — — — Core Outer diameter 39.1 39.1 36.3 36.339.1 36.3 36.3 39.1 (mm) Weight (g) 36.2 36.2 29.1 30.1 36.2 29.6 29.136.2 Intermediate Shore D hardness — — 47 58 — — 47 — layer Core Outerdiameter 39.1 39.1 39.1 39.1 39.1 36.3 39.1 39.1 or (mm) Core + Weight(g) 36.2 36.2 36.2 36.2 36.2 29.6 36.2 36.2 intermediate Deflectionunder 3.4 4.3 3.8 4.2 3.6 4.0 5.0 2.2 layer 100-kg load (mm) COR25 0.8350.857 0.839 0.859 0.826 0.842 0.839 0.831 COR50 0.712 0.720 0.703 0.7270.705 0.722 0.694 0.738 COR25-COR50 0.123 0.137 0.136 0.132 0.121 0.1200.145 0.093 Cover Shore D hardness 62 60 62 58 68 60 60 58 Gage (mm) 1.81.8 1.8 1.8 1.8 3.2 1.8 1.8 Ball COR25 0.860 0.860 0.857 0.858 0.8470.841 0.840 0.840 COR50 0.759 0.751 0.755 0.750 0.759 0.755 0.732 0.759COR25-COR50 0.101 0.109 0.102 0.108 0.088 0.086 0.108 0.081 Contact area(cm²) 5.45 5.67 5.50 5.70 5.24 5.38 5.97 4.91 W#1/HS35 Carry (m) 139.0138.5 138.5 137.0 133.0 133.5 132.0 131.0 Total (m) 155.0 156.2 154.0153.8 150.0 149.0 150.5 148.5 W#1/HS50 Carry (m) 237.0 237.5 235.4 235.0235.0 234.0 231.0 237.4 Total (m) 256.0 253.0 254.5 253.8 254.0 253.8243.5 256.5 Feel Good Good Good Good Fair Good Good Poor

[0046] As is evident from Table 1, the golf ball of Comparative Example1 travels a distance when hit at a high head speed, but travels shortwhen hit at a low head speed and gives an unsatisfactory feel since thecover is hard and the difference between COR25 and COR50 is outside therange of the invention. The golf ball of Comparative Example 2 exhibitspoor flight performance when hit at a low head speed since the cover isthick and the difference between COR25 and COR50 is outside the range ofthe invention. The golf ball of Comparative Example 3 exhibits poorflight performance when hit at a low head speed since the ball structureprior to formation of the cover is soft and the COR50 of the ball islower. The golf ball of Comparative Example 4 exhibits a poor feel anddoes not travel distance when hit at a low head speed since the ballstructure prior to formation of the cover is hard and the differencebetween COR25 and COR50 is outside the range of the invention.

[0047] In contrast, all the golf balls within the scope of the inventionare improved in flight performance both at low and high head speeds andgive a good feel.

[0048] There has been described a solid golf ball whose performancedepends little on the head speed. The flight performance of the ballwhen hit at a low head speed is improved while the flight performancewhen hit at a high head speed is little aggravated. Additionally, theball presents a good feel when hit.

[0049] Japanese Patent Application No. 11-094523 is incorporated hereinby reference.

[0050] Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A solid golf ball comprising a solid core and a cover, theimprovement wherein provided that a coefficient of restitution at afiring velocity of v m/s is designated by CORv, a ball structure priorto formation of the cover satisfies COR25-COR50≧0.100, and the ballsatisfies COR50≧0.740 and COR25-COR50 ≧0.09.
 2. The solid golf ball ofclaim 1 wherein the ball structure prior to formation of the coverundergoes a deflection of 2.8 to 5.0 mm under an applied load of 100 kg.3. The solid golf ball of claim 1 wherein the cover is formed of such amaterial that the Shore D hardness of the cover is not greater than327.61X+27.239 wherein X represents (COR25-COR50) of the ball structureprior to formation of the cover.
 4. The solid golf ball of claim 1wherein the cover has a gage of 1.2 to 3.0 mm.
 5. The solid golf ball ofclaim 1 having a contact area of 5.0 to 6.5 cm² when fired at a firingvelocity of 50 m/s.